With the advent of combination weighing systems throughput limitations for vertical form, fill and seal packaging (VFFS) machines may result from the limited cycle speed of their sealing jaw closure mechanisms. These mechanisms, such as disclosed in the commonly owned U.S. Pat. Nos. 4,537,012 and 4,040,237, are located in stationary or movable relation below the tube formers in the packaging machines. The sealing jaw mechanisms include a rotary actuator which moves links in opposite directions to open and close the sealing jaws forming the transverse seals in a thermoplastic tube of packaging material. Each sealing operation by the jaws forms the upper end seal of a filled package or bag and the lower end seal of the next package to be filled. An additional function that may be provided by the sealing jaws is that of "stripping" of the contents of the bag away from the transverse seal area. Unlike the bag sealing which mandates full closure of the sealing jaws, "stripping" of the contents of the bag requires only partial closure of the sealing mechanism and movement down the tube to urge the package contents away from the seal area.
Existing sealing mechanisms employ air actuator mechanisms which are only capable of either fully opened or fully closed jaw positioning. Therefore the sealing mechanism must be designed with sufficient jaw opening to accommodate the largest bag to be formed on that packaging machine. A typical ongoing VFFS machine is designed to accommodate packages from one-half ounce to twelve ounces in volume. A one-half ounce package has approximately a one inch flat package width and a tubular diameter of approximately 2.54 inches. Consequently, the sealing jaw opening need be only approximately 3.25 inches for this size package. In contrast, a 12 ounce package has a tube diameter of about 7 inches, which requires sealing jaw openings of between 7.5 and 8 inches. With package sealing jaw actuator mechanisms of the prior art, the sealing jaws are always cycled open to the maximum displacement which substantially reduces packaging cycle speeds and throughput for the smaller packages. Alternatively, the cyclic speed of the prior art machines is increased either by replacing the actuators with smaller actuators providing the appropriate jaw opening or by including mechanically adjusted or installed limit stops. With such alternative means, however, changing of the set up parameters required the machine to be removed from production at least temporarily while the mechanical alterations were made.
In order to accomplish the "stripping" of the package contents, prior art devices commonly employed two actuators, typically air cylinders, and other duplicate hardware to accomplish stepped sealing mechanism movement; first to a strip position and then to a seal position. The actuator's binary or two-state mode of operation (fully open jaws--fully closed jaws) also results in unwanted bouncing of the horizontal sealing jaws at the limits of jaw displacement. As a result, the sealing jaws themselves are worn prematurely while the overall operating speed of the packaging machine is reduced and package seal quality lowered.